OXIDATIVE DEHYDROGENATION OF BUTENES OVER ZINC-FERRITE CATALYSTS

Author

PARK, TAE-JIN

Date

1987

Degree

Doctor of Philosophy thesis

Abstract

The oxidative dehydrogenation (OXD) of butene over a zinc ferrite catalyst was studied in a batch recirculation and microcatalytic pulse reactor system at temperatures between 300 and 400$\sp\circ$C. Mechanistic features of the reaction were examined using deuterium labeled butene and $\sp{18}$O-labeled carbon dioxide experiments. Solid state changes in the catalyst were examined through x-ray powder diffraction and Mossbauer spectroscopy.
Reaction products consist of 1,3-butadiene, carbon dioxide, water, and butene isomers. At higher reaction temperatures, a small amount of carbon monoxide was observed. Kinetic expressions were constructed for both OXD and deep oxidation reactions based on a two site Langmuir-Hinshelwood model assuming dissociative adsorption for oxygen on one site and competitive adsorption by butene and butadiene on the other. Preliminary values for the kinetic parameters were evaluated from initial reaction rate data and were refined to fit the integral data obtained as a function of time. The activation energies for OXD and deep oxidation are 34.1 and 30.6 Kcal/mole, respectively.
The OXD of perdeuterated butene is slower than that of undeuterated butene, which indicates a significant kinetic isotope effect of 2.7 at 350$\sp\circ$C. A similar effect was observed for isomerization of cis-2-butene to 1-butene by double bond shift (1.4 at 350$\sp\circ$C). Apparently, carbon-hydrogen bond cleavage is involved in the rate limiting step for both reactions.
Microcatalytic pulse experiments carried out in the absence of gas phase oxygen indicated that the lattice oxygen does not participate in the reaction and that the migration of bulk oxygen to the surface of the catalyst is very slow under these conditions. A similar result was observed in $\sp{18}$O exchange experiments with C$\sp{18}$O$\sb 2$.
The experimental data are consistent with an oxidation-reduction cycle involving Fe$\sp{+2}$/Fe$\sp{+3}$ interconversion. Butene and oxygen molecules may adsorb into surface anion vacancies associated with an iron cation. The absence of intermolecular hydrogen-deuterium exchange during the isotopic tracer experiments indicates that the OXD and isomerization reaction rates are much greater than surface diffusion of hydrogen and/or deuterium.
The catalytic activity of zinc ferrite increases greatly when it is exposed to a high temperature under the reaction environment. Solid state studies using x-ray diffraction and Mossbauer spectroscopy indicated that the zinc ferrite phase decomposes into zinc oxide and iron oxides. Iron oxides thus formed are considered to be responsible for the enhancement of the catalytic activity.